One book I recommend for writers of secondary world fantasy is the Extinctosaurus. It is unfortunately out of print. But this is a great collection, with illustrations, of animals that have gone extinct over Earth’s history, including but definitely not limited to dinosaurs. If you are going to invent animals for your fantasy world, some of these creatures are a great place to start because you know they are evolutionarily possible. After all, they evolved once already.
When I wrote my Bronze Age fantasy, set in 3000 BC, one of my greatest delights was resurrecting a number of animals that were alive during that time period but are sadly gone now. I bent the rules a bit, moving the animals geographically–for example, the dwarf elephant I put in my novel never lived on the mainland. It was an island species. But this is the great fun of fantasy! We can cheat and make things up!
I altered a few species too. I made my onagers magical and, uh, I turned my blind river dolphins into scary man-eaters. That was mostly because it cracked me up. (DON’T FALL INTO THE RIVER! THE DOLPHINS WILL GET YOU!) It got a mixed reaction from my critique partners, so I don’t know if the killer dolphins will make it into the final draft.
But let’s talk about the onager. The onager, also known as the Syrian wild ass, is a relative of the horse that went extinct in the 20th century, mainly because it was hunted to death. Unlike other equids, its coloring changed with the seasons. It was famously untameable, which probably contributed to its demise.
The onager was one of the species I wanted to bring back in my Bronze Age book, although I decided to make it a little more exciting. I kept it a hunted species rather than a domesticated species but I exaggerated the color-changing feature and gave the onagers in my book the ability to change color not just with the seasons but on the fly. That is, as they were running, in order to confuse and outwit predators. And because of that ability, they’re the most high status prey animal hunted in that area, with hunters going out of their way to make an onager kill.